Axial flow fan assembly

ABSTRACT

An axial flow fan assembly having two or more fans in series to provide successive fan stages in which less than the total number of fans are of the variable pitch type.

I United States Patent [1 1 [111 3,768,546

Shipes [451 Oct. 30, 1973 4] AXIAL FLOW FAN ASSEMBLY 2,681,178 6/1954 Mart 261/D1G. 11

2,982,361 5/1961 R [75] Inventor: Kelly V. Shupes, Houston, Tex. 2,407,630 9/1946 22? [73] Assignees Hudson Products Corporation, 2,739,655 3/1956 Y---- Houston Tex 2,826,395 3/1958 Petty 165/39 [22] Filed, Dec 27 1971 FOREIGN PATENTS OR APPLICATIONS 357,368 9/1931 Great Britain 415/198 [21] Appl. No.: 212,555

OTHER PUBLICATIONS Publication: Pub in Aviation Week, 11-28-1960,

[52] US. Cl. 165/39, 415/130, Mfg/2976, variable Camber Propeller VOL 73, pages -51. [51] Int. Cl. F0ld 7/00, F281 27/00, Bh 1/20 [58] Field of Search 415/198, 199, 130, Primary Examiner Hemy F Raduazo 415/127, 149; 261/D1G. 10; 416/; Att0mey w F Hyer et al [56] References Cited [57] ABSTRACT An axial flow fan assembly having two or more fans in UNITED STATES PATENTS series to provide successive fan stages in which less 878,022 2/1908 Roper 416/127 than the total number of f are f the variable pitch 1,402,869 1/1922 Knuts0n.... 415/198 type 1,909,611 5/1933 Charavay.. 415/198 2,313,413 3/1943 Weske 415/198 6 Claims, 5 Drawing Figures /.9a F q /4 12 ll 1 /0 11' HI l- Z 4 PATENIED 00130 I975 3.768.546

SHEET 1 [1F 2 with respect to those of the first.

AXIAL FLOW FAN ASSEMBLY This invention relates to axial flow fan assemblies for use in air coolers or other industrial environments. More particularly, it relates to improvements in fan assemblies wherein two or more fans are mounted in series to provide successive stages within a single fan ring.

When used in air coolers, fan assemblies are mounted either above or below tube bundles for performing useful work by causing air to pass thereacross. In these and other industrial environments, the fans may be quite large, ranging in diameter from 6 to feet. The amount of work to be performed by a fan is approximately a function of the cube of its tip speed. At the same time, the noise generated by a fan is a function of its tip speed to the fifth power. Consequently, the problem of excessive noise is compounded as the work requirement on the fan assembly is increased. At the same time, of course, in response to public demand, our governmental bodies have, in regulatingnoise levels, adopted more stringent standards.

The historical purpose for using series fans has been to permit the useful application of more air moving power to a given piece of equipment than could be applied with only a single stage fan. In other cases, series fans have been used with the fan'blades rotating at somewhat lesser speeds so as to move the same quantity of air as a single stage fan, but with less noise.

However, in prior series axial flow fan assemblies of the latter category, the successive fan stages are of identical construction and axially spaced apart a considerable distance, in some cases as much as three fan diameters, because it has been shown that when such stages are not widely spaced apart, they do not perform as much useful work and are less sufficient. This large spacing between fan stages of these assemblies causes them to be quite expensive, and, in some cases, to interfere with available head room.

In a copending application Ser. No. 209,923, executed Dec. 17, 1971, entitled Axial Flow Fan Assembly, and assigned to the assignee of the present invention, there is disclosed a series axial flow fan assembly which is considerably less expensive to construct in that fan stages are considerably closer together than heretofore thought possible, and preferably substantially adjacent one another. More particularly, as explained in the copending application, it has been found that when the blades of the downstream fan have a greater average pitch angle or pitch than the blades of the upstream fan, such a fan assembly is capable of performing substantially the same useful work, and at substantially the same noise level, as the above-described prior series fan assemblies in which successive ,stages are widely spaced apart. As also explained in such application, it has further been discovered that additional advantageous results can be obtained when the blades of the second fan are circumferentially staggered or offset The environment in which fan assemblies are used often requires that the amount of air flow generated by the assembly be varied in view of changing conditions, such as a change in the ambient temperature or, in the case of an air cooler, a change in the temperature of fluid flowing through the tube bundles. More particularly, there is a need for varying the amount of air flow without shutdown of the operation of the fan assemblies. For this purpose, it has been proposed to provide fan assemblies with mechanisms for remotely adjusting the pitch of the blades of the fan assembly during their rotation.

A mechanism of this type is shown, for example, in Petty US. Pat. No. 2,826,395, wherein the blades of a fan assembly are caused to rotate within sockets on the hub of the fan in response to a controller, which may be at a remote location and actuated in response to a signal indicative of the temperature of a fluid flowing through a tube bundle. Thus, for example, upon a drop in the temperature of such fluid below that for which the system is designed, the mechanism may cause the blades of the fan to rotate into positions in which their pitch are reduced, so as to in turn cut down the amount of air flow across the bundle, and conversely, upon an increase in such temperature, the blades may be rotated into positions increasing their pitch so as to increase the amount of air flow.

Although fans having mechanism of this type, and known as variable pitch fans, serve an extremely useful purpose, they nevertheless are considerably more expensive and space consuming than conventional fan assemblies, known as fixed pitch fans, wherein the pitches of the blades can be adjusted, if at all, only by stopping rotation of the fan so that they may be manually rotated to a desired position. In fact, these mechanisms represent a large portion of the over-all cost of a single stage fan assembly. Obviously, in a series fan assembly having two or more fan stages with such mechanisms, they would represent an even larger percentage of the over-all cost of the assembly.

An object of this invention is to provide a series fan assembly in which the amount of air flow generated thereby can be varied without interruption of serivce, but in which the additional expense of providing this adjustment, as compared with a series fan having fixed pitch fans, is more economically feasible than heretofore thought possible.

This and other objects are accomplished, in accordance with the illustrated embodiment of the invention, by a series fan assembly in which less than all the stage fans are of the variable pitch type. That is, at least one fan is of the fixed pitch and at least another fan is of the variable pitch type, with additional fans, in the case of three or more stages, being of either type, whereby the amount of the air flow generated by the assembly is determined solely by the one or more variable pitch fans. Although the adjustment of the blades of one fan to a pitch different from that of another may result in some loss of efficiency, and thus an increased cost of operation, as compared with a fan assembly in which all fans are of the variable pitch type adapted to be adjusted to the same extent, this is ordinarily more than compensated for by the reduced initial investment. Thus, in the range of pitch adjustments which would ordinarily be required in the operation of the variable pitch fan or fans of a fan assembly for use as part of an air cooler, for example, there is little or no loss of efficiency. Still further, as suggested by the aforementioned copending patent application, the fan whose blades are adjusted to the larger pitch during design conditions 'may be disposed downstream of the other, and the fans arranged axially close together, so as to further reduce the cost of the fan assembly without substantial loss of efficiency, as compared with conventional fan assemblies having widely spaced stages.

In the drawings, wherein like reference characters are used throughout'to designate like parts:

FIG. 1 is an elevational view of an air cooler which has been broken away in part to show a tube bundle and a fan assembly constructed in accordance with one embodiment of the present invention and supported above the bundle for drawing air upwardly thereacross;

FIGS. 2 and 3 are elevational views of a portion of air coolers having fan assemblies constructed in accordance with additional embodiments of the invention;

FIG. 4 is a graph showing a curve illustrating the amount of air flow which must be generated by a fan assembly in order to condense a process fluid in an air cooler, such as that shown in FIG. 1, at different ambient temperature conditions; and

FIG. 5 is another graph showing curves illustrating in solid and broken lines, respectively, the power required to so condense the fluid by a fan assembly in which both stages are variable pitch fans and a fan assembly in which one stage is a variable pitch fan and the other is a fixed pitch fan.

With reference now to the above-described drawings, the air cooler shown in FIG. 1, and designated in its entirety by reference character 10, includes a tube bundle 11 mounted on vertical columns 12 above the surface 13, and a series fan assembly 14 mounted above the tube bundle by means of a transition 15. As shown by the broken away portion of FIG. 1, the bundle 11 includes a plurality of heat exchange tubes 16 extending laterally between headers (not shown) at opposite ends -of the bundles for conducting a process fluid to be cooled across the air stream induced in an upward direction by means of the fan assembly. Side walls 17 extend along opposite sides of the tube bundle from one header to the other so as to confine air flow to the bundle.

The fan assembly 14 includes a cylindrical fan ring having upstream and downstream series fans 19 and 20, respectively, providing successive stages mounted for rotation coaxially thereof. More particularly, the fans are of such diameter as to cause the tips of their blades 19a and 20a to move closely and concentrically within the fan ring. Also, and as shown in FIG. 1, the blades of both fans are adjusted to positive pitch to cause air to move upwardly through the fan ring, and thus upwardly across the tube bundle in response to rotation of the fans in clockwise direction (looking downwardly). It is in this sense i.e., direction of air movement that the lower fan 19 is called upstream and the upper fan 20 is called downstream." However, it is contemplated that, as discussed to follow, the blades of one and possibly both such fans may be adjusted to cause a reversal of air flow i.e., in a direction downwardly through the fan ring. 5

Both fans are mounted on a shaft 21 which extends vertically and coaxially of the fan ring. The lower end of the shaft is driven by a motor 22 mounted on a motor support 23 suspended from the tube bundle or other portion of the air cooler in any suitable manner. The motor drives a belt within abelt guard 24 disposed about the lower end of the shaft for rotating the fans at a desired speed. The shaft is mounted for rotation at its upper end by means of a bearing 21a supported in the fan ring 18 by radial struts 21b.

The lower or upstream fan 19 includes a hub 25 fixed to shaft 21 and having a plurality of blade sockets 26 extending radially in equally spaced apart relation. The inner ends of the blades 19a are releasably secured in the hubs, which enables the average pitch on each blade to be adjusted as desired, depending on operating conditions. However, such adjustment requires that the fan first be stopped, and, in this sense, the fan 19 is of the fixed pitch type.

The upper or downstream fan 20 also includes a hub 27 fixed to shaft 21 and having a plurality of sockets 28 extending radially therefrom to receive the inner ends of blades 20a. However, the hub 27 includes a mechanism (not shown) such as that shown in US. Pat. No. 2,826,395, which is remotely operable for causing the blades to rotate in their sockets, and thus adjust their pitch, in response to a signal indicative of a condition in the cooler. In this sense, the fan 20 is of the variable pitch type. As disclosed in such prior patent, there is a pressure responsive operator at the upper end of the hub 27 which is adapted to receive a signal, which may represent the temperature of process fluid in the bundle 16. This signal may be produced by a transducer 30 in a conduit connecting the outlet end of the bundle with the operator. The operator is also adapted to receive power fluid supplied through a conduit 31 for operating the mechanism, and thus rotating the blades 20a, in a desired manner responsive to such signal. Thus, for example, upon an increase in the temperature of the process fluid, the mechanism may cause the pitch of the blades 20a to increase and thus increase the amount of air flow across the bundle. On the other hand, upon a decrease in the temperature of the process fluid, the blade pitches may be decreased to decrease the amount of such air flow.

The hubs of the fans 19 and 20, and thus the planes of the inner sides of the fans themselves, are substantially adjacent one another, whereby the axial distance between the fans is at substantially a minimum. Also, the blades of the fans are shown in so-called design or percent air flow position, wherein the positive pitch of those of the downstream fan 20 is greater than that of the blades of the upstream fan 19, the particular pitches and the axial spacing of the blades being determined in accordance with the copending application. As will be apparent from FIG. 1, the blades of the two fans are staggered or circumferentially offset from one another, the extent of such staggering being determined inacc'ordance with the teaching of the copending application.

As shown, each blade tapers inwardly in a radially outward direction, and has a cross-section which is of generally air foil shape. In some cases, the opposite surfaces of the blades may twist to some extent, so that the pitch, or angle which the active or upper blades face forms with a horizontal plane perpendicular to the axis of the shaft, may vary to some extent along the length of the blade, and it is in this sense that the term average pitch is used herein. However, as is well known in the art, this variance is generally relatively small and thus insignificant insofar as design considerations are concerned.

As previously mentioned, during operation of the fan assembly, the pitch of the blades of the fan 20 may be adjusted so as to change the amount of air flow through the fan ring and thus across the tube bundle l6, and, in some cases, the direction of such air flow. As previously mentioned, the blades 20a of the fan 20 are shown in FIG. I adjusted to a positive pitch greater than the positive pitch of the blades 19a of the fan 19, for generating 100 percent air flow during conditions for which the assembly is designed. Adjustment of the blades 20a from a positive pitch equal to the positive pitch of the blades 19a to a feathering point will usually not decrease the amount of air flow upwardly through the fan ring. In other words, within this range, the amount of air flow through the fan ring will be determined by the pitch of the blades 19a. However, in the event it is desired to reduce the amount of air flow through the fan ring, the pitch of the blades 20a may actually be adjusted to a negative pitch so as to oppose the direction of air flow generated by the blades 19a. In fact, it is contemplated that the blades 20a of the variable pitch fan 20 may be adjusted to negative pitches greater than the positive pitch of the blades 19a so as to actually cause air flow in a direction downwardly through the fan ring.

As previously mentioned, under ordinary design conditions, air flow through the fan ring will be in an upward direction and in an amount determined by adjustment of the blades 19a to a positive pitch greater than that of the blades 20a. According to the present invention, it is preferred that, as illustrated in FIG. 1, the variable pitch fan 20 be on the downstream side of the fixed pitch fan 19, because it is known that the amount of power required to rotate a fan is dependent upon the extent to which its blades are pitched i.e., the greater the pitch, the greater the power requirements. Consequently, it is possible to take advantage of the lower power requirements when the blades of the variable pitch fan are adjusted to lesser pitches during changes from design conditions.

The particular curve shown in FIG. 4 is representative of the amount of air flow relative to design air flow which must be generated across a tube bundle, such as that shown in FIG. 1, in order to condense a vapor at 190 F with ambient air less than 90 F. Thus, the term design is used to designate the anticipated condition, wherein the ambient temperature is 90 F and the blades of the fans are adjusted as described to generate 100 percent air flow across the tube bundle through which the vapor is conducted. As shown in the curve, and as is wellknown in the art, as the ambient temperature decreases, the'percentage of design air flow required to condense the vapor decreases. This decrease in'air flow is, of course, effected by an adjustment of the blades 20a of the'fan 20 to a pitch for reducing the amount of air flow which would be generated by the fan 19 alone. This adjustment is, of course, automatically responsive to a signal produced in transducer 30 in response to a decrease in the outlet temperature of the process fluid, which of course results from a decrease in ambient temperature.

The solid and broken linecurves of FIG. 5 compare the power required by a fan assembly constructed as shown in FIG. 1, and a fan assembly (not shown) forming part of the same air cooler of FIG. 1, but wherein both fans are variable pitch type i.e., of the construction of the fan 20. As will be apparent from FIG. 5, the latter fan assembly would be more efficient when the air cooler is operating at an ambient temperature less than approximately 62 F, at which point the illustrated fan assembly of the present invention begins to subtract from the amount of air flow due to the fixed pitch fan 19 alone. However, this invention takes advantage of the fact that during the majority of the time of its use, the air cooler will be operating at an ambient temperature greater than the point of divergence of the two curves, and in an even greater percentage of the time,

during such time that the power requirements of the comparative fan assemblies are either the same or not substantially different.

For example, for the years l95l through 1960, the northern and southern U. S. cities of Philadelphia and Miami have average temperatures 15 to 40 F only 12 pecent of the time, average tempertures between 40 and 65 F only 26 percent of the time, and average temperatures between 65 and 90F during the remaining 62 percent of the time. As shown by the curve of FIG. 4, it is, of course, during this latter temperature range that the two fan assemblies would operate with approximately the same efficiency.

By striking an average, it has been found that the fan assembly constructed in accordance with the present invention will consume 59 percent of the designed power rate over the year, while a fan assembly having variable pitch fans at both stages will consume 48 percent of the designed power rate during the year. Based on typical figures of a 25 horsepower requirement per fan (at design level) and power costs of $60.00 per horsepower per year, power costs per fan for the fan assembly of the present invention will be $885.00 per year, as compared with the power cost per fan of the fan assembly having variable pitch fans at both stages will be $720.00 per year.

On the other hand, in a typical fan assembly wherein both fans are 14 feet in diameter and have six blades each, the cost of the fan assembly of the present invention would be approximately $3,540.00, as compared with a cost of approximately $4,774.00 for the fan assembly having variable pitch fans at both stages. Thus, taking the difference in power costs between the two fan assemblies, it would take approximately 7% years of savings and operating expenses to pay the capital investment difference between the fan assembly of the present invention and the comparative fan assemblies.

It may be found that the difference in operating costs of the above-described fan assemblies may be reduced even further. Thus, several such fan assemblies are normally required in a given installation, and, in accordance with well known practices, it may be possible to effect a desired reduction in the amount of design air flow, responsive to a change in operating conditions, by shutting off one or more of the assemblies. In this event, the blades of each of the downstream fans of the fan assemblies remaining in operation would be adjusted toward a feathering pitch'to a lesser extent than would be required if the same amount of air was to be moved with all the fan assemblies remaining in service.

As a result, the power required to operate the fewer number of fan assemblies would be less than that required to operate all of them. 7

By way of example, assume that the air cooler of FIG. 1 has four fan assemblies, and that, in the exemplary system for which the curves of FIGS. 4 and 5 are applicable, the ambient temperature drops to 30 F. From FIG. 4, it is seen that, at this temperature, the required air flow is 48 percent of design, or I92 percent (4 fans X 48 percent) for all four fans; and from FIG. 5, it can be seen that the total power requirement is 248 percent (62 percent X 4 fans). If one fan assembly is cut out, the required percentage of design air flow through each of the remaining three is 192 percent/3 or 64 percent; and, from FIG. 4, it-can be determined that this is the equivalent of all four fans operating at an ambient temperature of 57 F. Referring then to FIG. 5, it will be seen that the power requirement of each of the three assemblies is 40 percent of design, and thus equivalent to 160 percent (4 X 40 percent) of design, as compared with 192 percent with all four fans operating.

In the embodiment of the invention illustrated in FIG. 2, the fan assembly has three stages, including not only the fixed pitch fan 19 and the variable pitch fan 20 downstream thereof, but also an intermediate or second stage fixed pitch fan 19'. As shown in FIG. 2, this second stage fan 19 may be of identical construction to the first fixed pitch fan 19, and in accordance with the teachings of the copending patent application, the blades of the second fan 19 are substantially adjacent to and adjusted to a positive pitch greater than those of the first fan. As in the case of the first and second fans of the FIG. 1 embodiment, during design conditions, the blades of the fan 20 are substantially adjacent those of the fan 19' and adjusted to a positive pitch greater than the positive pitch of the blades 19a thereof. Still further, and as also shown in FIG. 2, the blades of the second fan 19' are staggered or circumferentially offset with respect to the first fan 19, and the blades of the third fan 20 are staggered or circumferentially offset with respect to the blades of the second fan 19'. The difference in pitches between adjacent fans, as well as the staggering or offsetting of their blades, may be determined in accordance with the teachings of the copending patent application.

The operation of the fan assembly of FIG. 2 will be obvious in the light of the foregoing discussion of the operation of the fan assembly of FIG. 1. Thus, for example, upon a drop in ambient temperature, the pitch of the blades 20a of the variable pitch fan 20 would be reduced so as to reduce the amount of air flow through the fan assembly. Thus, it is contemplated that the variable pitch mechanism of hub 27 of the fan 20 would permit the pitch of the blades of the fan 20 to be adjusted over a range from a positive pitch greater than the positive pitch of the blades 19a to a positive pitch less than the positive pitch of the blades 19a.

The embodiment of the fan assembly illustrated in FIG. 3 also has three fan stages, and similarly to the fan assembly of FIG. 2, the fan 19 of the lower or first stage thereof i.e., upstream with respect to the direction of design air flow is of the fixed pitch type which may be identical to the fan 19 of the assembly of FIG. 1. Still further, the second fan 20 downstream of the first fan is of a variable pitch type which may be identical in construction and operation to the variable pitch fan 20 of the FIG. 1 fan assembly. Thus, the fan 20 is substantially adjacent the fixed pitch fan 19, and, under design conditions, its blades are adjusted to a pitch greater than the positive pitch of the blades 19a of the fixed pitch fan 19, and are further circumferentially offset or staggered with respect thereto.

However, the third fan 20 of the FIG. 3 fan assembly is of the variable pitch type, which maybe identical in construction to the variable pitch fan 20 of each of the FIGS. 1, 2 and 3 embodiments. Although the fan 20 is axially spaced from the second fan 20 a greater distance than the distance between the fans 19 and 20 (in order to accommodate the upward extension of the hub 27 of the fan 20), the fans 20 and 20' are nevertheless relatively close together, in comparison to adjacent fans of prior series fan assemblies. In accordance with the teachings of the copending application, under design conditions, the positive pitch of the blades 20a will be greater than the positive pitch of the blades 20a, and the blades 20a and 20a are circumferentially offset or staggered with respect to one another. More particularly, similarly to the blades of the fan 20, the blades 20a of the variable pitch fan 20' are adjustable, depending on system conditions, over a range between a pitch greater than the pitch of the fan blades 19a and a pitch less than the pitch of the fan blades 19a.

It is contemplated that a reduction in the amount of air flow through the fan assembly of FIG. 3 may be effected by adjustment of the pitch of the blades of one or both of the variable pitch fans 20 and 20'. Thus, the mechanisms for so adjusting the pitch of the blades of each such fan may be responsive to a single transducer, such as transducer 30 shown in FIG. 1, or separate transducers, in which event each separate transducer may be responsive to a different signal. Also, the power fluid for operating each such mechanism may come from the same or different sources.

From the foregoing it will be seen that this invention is one well adapted to attain all of the ends and objects hereinabove set forth, together with other advantages which are obvious and which are inherent to the apparatus.

It will be understood that certain features and subcombinations are of utility and may be employed without reference to other features and subcombinations. This is contemplated by and is within the scope of the claims.

As many possible embodiments may be made of the invention without departing from the scope thereof, it is to be understood that all matter herein set forth or shown in the accompanying drawings is to be interpreted as illustrative and not in a limiting sense.

The invention having been described, what is claimed 1. An air cooler, comprising a tube bundle, and an axial flow fan assembly mounted on one side of the tube bundle for causing air to pass thereacross, said fan assembly including a fan ring, first and second axial flow fans mounted coaxially within the fan ring and relatively close together, means for rotating the first and second fans, the blades of the first fan having a positive pitch which is non-adjustable during rotation thereof and arranged to move the air through the fan ring in a direction from said first fan to said second fan, and remotely operable means for adjusting the pitch of the blades of the second fan, during rotation thereof, within a range between positive pitches gerater than and less than the positive pitch of the blades of the first fan.

2. An air cooler of the character defined in claim 1, wherein said remotely operable pitch adjusting means includes means for adjusting the blades of the second fan to a negative pitch.

3. An air cooler, comprising a tube bundle, and an axial flow fan assembly mounted on one side of the tube bundle for causing air to pass thereacross, said fan assembly including a fan ring, first, second and third axial flow fans mounted coaxially within the fan ring, the first fan being relatively close to the second fan, and the second fan being relatively close to the third fan, means for rotating the first, second and third fans, the blades of the first fan having a positive pitch which is non-adjustable during rotation thereof and arranged to move the air through the fan ring in a direction from said first fan to said second fan, and remotely operable means for adjusting the pitch of the blades of each of the second and third fans, during rotation thereof, within a range between positive pitches greater than and less than the positive pitch of the blades of the first fan.

4. An air cooler of the character defined in claim 3, wherein said remotely operable pitch adjusting-means includes means for adjusting the blades of the second and third fan to a negative pitch.

5. An air cooler, comprising a tube bundle, and an axial flow fan assembly mounted on one side of the tube bundle for causing air to pass thereacross, said fan assembly including a fan ring, first, second and third axial flow fans mounted coaxially within the fan ring, the third fan being relatively close to the second fan, and the second fan being relatively close to the third fan, means for rotating the first, second and third fans,

the blades of each of the first and second fans having a postivie pitch which is non-adjustable during rotation thereof and arranged to move air through the fan ring in a direction from the first fan to the second fan and from the second fan to the third, with the pitch of the blades of the second fan being greater than the pitch of the blades of the third fan, and remotely operable means for adjusting the pitch of the blades of the third fan, during rotation thereof, within a range between the positive pitches greater than and less than the positive pitch of the second fan.

6. An air cooler of the character defined in claim 5, wherein said remotely operable pitch adjusting means includes means for adjusting the blade of the third fan to a negative pitch. 

1. An air cooler, comprising a tube bundle, and an axial flow fan assembly mounted on one side of the tube bundle for causing air to pass thereacross, said fan assembly including a fan ring, first and second axial flow fans mounted coaxially within the fan ring and relatively close together, means for rotating the first and second fans, the blades of the first fan having a positive pitch which is non-adjustable during rotation thereof and arranged to move the air through the fan ring in a direction from said first fan to said second fan, and remotely operable means for adjusting the pitch of the blades of the second fan, during rotation thereof, within a range between positive pitches gerater than and less than the positive pitch of the blades of the first fan.
 2. An air cooler of the character defined in claim 1, wherein said remotely operable pitch adjusting means includes means for adjusting the blades of the second fan to a negative pitch.
 3. An air cooler, comprising a tube bundle, and an axial flow fan assembly mounted on one side of the tube bundle for causing air to pass thereacross, said fan assembly including a fan ring, first, second and third axial flow fans mounted coaxially within the fan ring, the first fan being relatively close to the second fan, and the second fan being relatively close to the third fan, means for rotating the first, second and third fans, the blades of the first fan having a positive pitch which is non-adjustable during rotation thereof and arranged to move the air through the fan ring in a direction from said first fan to said second fan, and remotely operable means for adjusting the pitch of the blades of each of the second and third fans, during rotation thereof, within a range between positive pitches greater than and less than the positive pitch of the blades of the first fan.
 4. An air cooler of the character defined in claim 3, wherein said remotely operable pitch adjusting means includes means for adjusting the blades of the second and third fan to a negative pitch.
 5. An air cooler, comprising a tube bundle, and an axial flow fan assembly mounted on one side of the tube bundle for causing air to pass thereacross, said fan assembly including a fan ring, first, second and third axial flow fans mounted coaxially within the fan ring, the third fan being relatively close to the second fan, and the second fan being relatively close to the third fan, means for rotating the first, second and third fans, the blades of each of the first and second fans having a postivie pitch which is non-adjustable during rotation thereof and arranged to move air through the fan ring in a direction from the first fan to the second fan and from the second fan to the third, with the pitch of the blades of the second fan being greater than the pitch of the blades of the third fan, and remotely operable means for adjusting the pitch of the blades of the third fan, during rotation thereof, within a range between the positive pitches greater than and less than the positive pitch of the second fan.
 6. An air cooler of the character defined in claim 5, wherein said remotely operable pitch adjusting means includes means for adjusting the blade of the third fan to a negative pitch. 